Correlation techniques are frequently employed in ranging and depth finding systems. Of particular interest in the sonar depth sounding situation is a correlator operating with one bit samples of the received echo. A one bit sampler, or hard limiter, precedes the correlator as is disclosed in the U.S. Pat. No. 3,786,405 which issued to M. A. Chramiec and W. L. Konrad on Jan. 15, 1974. Such a correlator is particularly useful in sonar sounding applications since it readily accomplishes a correlation between a phase modulated signal and a phase modulated reference and has great simplicity of structure as compared to a correlator which is capable of processing multiple bit samples of data. Such a correlator gives a clear output signal in response to echoes received from flat bottoms in the ocean.
A problem arises in the situation wherein the aforementioned one bit correlator is utilized in depth sounding situations wherein the ocean bottom is sloping with angles of slope in excess of 5.degree.. As will be explained hereinafter, a reflecting surface, such as the sloping bottom, which is inclined relative to the direction of propagation of a beam of sound from a sonar transmitter produces a train of echoes occurring in rapid succession. Where the time intervals between these rapidly occurring echoes is much less than a time interval equal to the reciprocal of the correlator bandwidth, numerous echo pulses appear at the output of the correlator and each of these output pulses has amplitudes substantially reduced from that of a single output pulse as would be produced in response to an echo from a flat bottom under identical conditions of a signal-to-noise ratio. As a result, it is difficult for an operator to determine which of the rapidly occurring reduced amplitude pulses represents the depth of the ocean bottom and, there is a similar difficulty involved in the implementation of an automatic depth tracking circuit.